1. Field of the Invention
This invention generally relates to an ink jet printing system, and in particular, to such an ink jet printing system in which air bubbles trapped in the ink inside an ink chamber from which ink is discharged in the form of ink droplets for printing are advantageously removed from the ink chamber to insure constant ink discharging performance at all times.
2. Description of the Prior Art
Various types of ink jet printing systems have heretofore been proposed. In one type of the ink jet printing system, ink droplets are continuously discharged out of a printing head whereby the ink droplets are uniformly charged before being passed between a pair of deflecting electrodes to which signal pulses are applied or the ink droplets are individually charged to the amount in accordance with an image signal before being passed through a pair of uniformly biased deflecting plates. There is another type of the ink jet printing system, which is often referred to as the on-demand type ink jet printer, and, in this type, ink droplets are formed as the ink is discharged out of the ink chamber when the volume inside the ink chamber is reduced, for example, by deflecting a part of the wall defining the ink chamber. In the former case, the ink is discharged out of the ink chamber or nozzle by applying pressure to the ink; on the other hand, in the latter case, the ink is discharged out of the ink chamber by means of volume displacement.
However, if air bubbles are trapped inside the ink, particularly that portion of the ink contained inside the ink chamber, such an air-bubble-containing ink as a whole comes to exhibit compressibility thereby adversely affecting the ink discharge performance. One can think of various causes of such air bubble entrapment; for example, air bubbles may be introduced into the ink inside the ink chamber through the mouth of the ink nozzle which defines the ink chamber, or air bubbles may be generated in the ink as the temperature changes.
FIG. 1 shows in cross-section the ink discharging head to be used in the on-demand type ink jet printing system. As shown, the ink discharging head comprises a nozzle 1 defining an ink chamber 4 for containing a quantity of ink therein. A mouth or ink discharging hole 2 is defined at the forward end of the nozzle 1 through which the ink inside the ink chamber 4 is discharged out into the air as targeted toward a recording medium positioned opposite to the printing head. The nozzle 1 is also provided with another hole in which a pressure member 3 is mounted with its periphery fixed to the circumference of the hole, so that the pressure member 3 constitutes a part of the wall which defines the ink chamber 4. The pressure member 3 is usually formed by overlaying a piezoelectric plate as adhered onto a support plate. Also shown in FIG. 1 is an ink supplying tube 5 which is connected to the inlet side of the nozzle 1.
In operation of the discharging head shown in FIG. 1, electrical pulse signals are applied to the piezoelectric plate of the pressure member 3, so that the pressure member 3 deflects inwardly of the ink chamber 4 in accordance with the applied pulses thereby the ink inside the chamber 4 is pressurized and discharged out into the air through the mouth 2. In this instance, however, if air bubbles 6 are present inside the ink contained in the ink chamber 4, as shown in FIG. 2, the ink which is normally considered incompressible in such application begins to exhibit compressibility due to the existence of air bubbles therein. Under such circumstances, the ink discharging characteristics are severely impaired or no ink is discharged at all even if the pressure member 3 is inwardly deflected in an extreme case.
Therefore it is extremely important to make the ink free of air bubbles in order to insure ink discharging performance in an ink jet printing system. In this regard, several approaches have been proposed to prevent the entrapment of air bubbles during operation or to remove the air bubbles somehow trapped in the ink. However, none of the prior art approaches is satisfactory for various reasons. For example, some prior art approaches only propose to prevent entrapment of air bubbles from the supply side of the ink chamber and others require complicated and bulky devices. Thus there has been a need for the advent of a new approach to cope with the problem of air bubble entrapment into the ink in an ink jet printing system.